/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements.  See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License.  You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package javax.servlet.http;

import java.util.Enumeration;
import javax.servlet.ServletContext;

/**
 * HttpSession是与Cookie对应的服务端技术，在服务端首次应答客户端请求时，会首先在服务端生成HttpSession，并存储在服务端存储中。
 * 然后将HttpSession的id放入响应头Set-Cookie中，跟随响应一起发送给客户端，客户端存储获得的Cookie，随下次请求一起发送给服务端。
 * 服务端根据Cookie中的id值查找对应的HttpSession，确定会话有效性。
 *
 *
 * 提供一种方法，用于跨多个页面请求或访问网站来标识用户，并存储有关该用户的信息.
 * Servlet容器使用此接口在HTTP客户端和HTTP服务器之间创建会话. 会话在用户的多个连接或页请求中持续指定的时间段.
 * 一个会话通常对应于一个用户，该用户可以多次访问一个站点. 服务器可以通过多种方式维护会话，例如使用cookies或重写url.
 * 这个接口允许servlet查看和操作会话的信息，例如会话标识符、创建时间和上次访问时间将对象绑定到会话，从而允许用户信息在多个用户连接之间保持不变.
 * 当应用程序在会话中存储对象或从会话中删除对象时，会话将检查该对象是否实现HttpSessionBindingListener.
 * 如果是这样，servlet会通知对象它已绑定到会话或从会话中取消绑定.在绑定完成后发送通知.对于无效或过期的会话，将在会话失效或过期后发送通知.
 * 当容器在分布式容器设置中的vm之间迁移会话时，将通知实现HttpSessionActivationListener接口的所有会话属性.
 * 会话信息的作用域仅限于当前的web应用程序（ServletContext），因此存储在一个上下文中的信息在另一个上下文中不会直接可见.
 *
 *
 * Provides a way to identify a user across more than one page
 * request or visit to a Web site and to store information about that user.
 *
 * <p>The servlet container uses this interface to create a session
 * between an HTTP client and an HTTP server. The session persists
 * for a specified time period, across more than one connection or
 * page request from the user. A session usually corresponds to one
 * user, who may visit a site many times. The server can maintain a
 * session in many ways such as using cookies or rewriting URLs.
 *
 * <p>This interface allows servlets to
 * <ul>
 * <li>View and manipulate information about a session, such as
 *     the session identifier, creation time, and last accessed time
 * <li>Bind objects to sessions, allowing user information to persist
 *     across multiple user connections
 * </ul>
 *
 * <p>When an application stores an object in or removes an object from a
 * session, the session checks whether the object implements
 * {@link HttpSessionBindingListener}. If it does,
 * the servlet notifies the object that it has been bound to or unbound
 * from the session. Notifications are sent after the binding methods complete.
 * For session that are invalidated or expire, notifications are sent after
 * the session has been invalidated or expired.
 *
 * <p> When container migrates a session between VMs in a distributed container
 * setting, all session attributes implementing the {@link HttpSessionActivationListener}
 * interface are notified.
 *
 * <p>A servlet should be able to handle cases in which
 * the client does not choose to join a session, such as when cookies are
 * intentionally turned off. Until the client joins the session,
 * <code>isNew</code> returns <code>true</code>.  If the client chooses
 * not to join
 * the session, <code>getSession</code> will return a different session
 * on each request, and <code>isNew</code> will always return
 * <code>true</code>.
 *
 * <p>Session information is scoped only to the current web application
 * (<code>ServletContext</code>), so information stored in one context
 * will not be directly visible in another.
 *
 * @author	Various
 *
 *
 *
 * @see 	HttpSessionBindingListener
 * @see 	HttpSessionContext
 *
 */

public interface HttpSession {




    /**
     * 返回Session创建时间.自1970年1月1日午夜起以毫秒计.
     *
     * Returns the time when this session was created, measured
     * in milliseconds since midnight January 1, 1970 GMT.
     *
     * @return				a <code>long</code> specifying
     * 					when this session was created,
     *					expressed in
     *					milliseconds since 1/1/1970 GMT
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public long getCreationTime();




    /**
     * 返回一个字符串，该字符串包含分配给此会话的唯一标识符.
     * 标识符由servlet容器分配，并且依赖于实现.
     * Returns a string containing the unique identifier assigned
     * to this session. The identifier is assigned
     * by the servlet container and is implementation dependent.
     *
     * @return				a string specifying the identifier
     *					assigned to this session
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public String getId();




    /**
     * 返回会话的最后访问时间，即自1970年1月1日午夜起的毫秒数，并用容器接收请求的时间标记.
     * 应用程序执行的操作，例如获取或设置与会话关联的值，不影响访问时间.
     * Returns the last time the client sent a request associated with
     * this session, as the number of milliseconds since midnight
     * January 1, 1970 GMT, and marked by the time the container received the request.
     *
     * <p>Actions that your application takes, such as getting or setting
     * a value associated with the session, do not affect the access
     * time.
     *
     * @return				a <code>long</code>
     *					representing the last time
     *					the client sent a request associated
     *					with this session, expressed in
     *					milliseconds since 1/1/1970 GMT
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public long getLastAccessedTime();


    /**
     * 返回此会话所属的ServletContext.
     *
     * Returns the ServletContext to which this session belongs.
     *
     * @return The ServletContext object for the web application
     * @since 2.3
     */

    public ServletContext getServletContext();


    /**
     * 指定在Servlet容器使此会话无效之前，客户端请求之间的时间（以秒为单位）.负时间表示会话不应超时.
     *       也就是所谓的会话超时时间.
     *
     * Specifies the time, in seconds, between client requests before the
     * servlet container will invalidate this session.  A negative time
     * indicates the session should never timeout.
     *
     * @param interval		An integer specifying the number
     * 				of seconds
     *
     */

    public void setMaxInactiveInterval(int interval);




   /**
    * 获取会话超时时间.
    *
    * Returns the maximum time interval, in seconds, that
    * the servlet container will keep this session open between
    * client accesses. After this interval, the servlet container
    * will invalidate the session.  The maximum time interval can be set
    * with the <code>setMaxInactiveInterval</code> method.
    * A negative time indicates the session should never timeout.
    *
    *
    * @return		an integer specifying the number of
    *			seconds this session remains open
    *			between client requests
    *
    * @see		#setMaxInactiveInterval
    *
    *
    */

    public int getMaxInactiveInterval();




   /**
    * 获取HttpSessionContext对象.
    * @deprecated 	As of Version 2.1, this method is
    *			deprecated and has no replacement.
    *			It will be removed in a future
    *			version of the Java Servlet API.
    *
    */

    public HttpSessionContext getSessionContext();




    /**
     * 返回在此会话中使用指定名称绑定的对象，如果名称下未绑定任何对象，则返回null.
     * Returns the object bound with the specified name in this session, or
     * <code>null</code> if no object is bound under the name.
     *
     * @param name		a string specifying the name of the object
     *
     * @return			the object with the specified name
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public Object getAttribute(String name);




    /**
     * 使用getAttribute代替此方法.
     * @deprecated 	As of Version 2.2, this method is
     * 			replaced by {@link #getAttribute}.
     *
     * @param name		a string specifying the name of the object
     *
     * @return			the object with the specified name
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public Object getValue(String name);




    /**
     * 返回String对象的枚举，其中包含绑定到此会话的所有对象的名称.
     * Returns an <code>Enumeration</code> of <code>String</code> objects
     * containing the names of all the objects bound to this session.
     *
     * @return			an <code>Enumeration</code> of
     *				<code>String</code> objects specifying the
     *				names of all the objects bound to
     *				this session
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public Enumeration getAttributeNames();




    /**
     * 使用getAttributeNames代替此方法.
     * @deprecated 	As of Version 2.2, this method is
     * 			replaced by {@link #getAttributeNames}
     *
     * @return				an array of <code>String</code>
     *					objects specifying the
     *					names of all the objects bound to
     *					this session
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public String[] getValueNames();




    /**
     * 使用指定的名称将对象绑定到此会话.如果已将同名对象绑定到会话，则将替换该对象.
     * 此方法执行后，如果新对象实现HttpSessionBindingListener，则容器调用HttpSeSessionBindingListener.valueBound.
     * 容器然后通知web应用程序中的任何HttpSessionAttributeListener.
     * 如果一个对象已经绑定到实现HttpSessionBindingListener的此名称的会话，则其HttpSeSessionBindingListener.valueUnbound方法被调用.
     * 如果传入的值为null，则与调用removeAttribute()具有相同的效果.
     *
     * Binds an object to this session, using the name specified.
     * If an object of the same name is already bound to the session,
     * the object is replaced.
     *
     * <p>After this method executes, and if the new object
     * implements <code>HttpSessionBindingListener</code>,
     * the container calls
     * <code>HttpSessionBindingListener.valueBound</code>. The container then
     * notifies any <code>HttpSessionAttributeListener</code>s in the web
     * application.

     * <p>If an object was already bound to this session of this name
     * that implements <code>HttpSessionBindingListener</code>, its
     * <code>HttpSessionBindingListener.valueUnbound</code> method is called.
     *
     * <p>If the value passed in is null, this has the same effect as calling
     * <code>removeAttribute()</code>.
     *
     *
     * @param name			the name to which the object is bound;
     *					cannot be null
     *
     * @param value			the object to be bound
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public void setAttribute(String name, Object value);





    /**
     * 使用setAttribute代替此方法.
     * @deprecated 	As of Version 2.2, this method is
     * 			replaced by {@link #setAttribute}
     *
     * @param name			the name to which the object is bound;
     *					cannot be null
     *
     * @param value			the object to be bound; cannot be null
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     *
     */

    public void putValue(String name, Object value);





    /**
     * 从此会话中移除使用指定名称绑定的对象.如果会话没有使用指定名称绑定的对象，则此方法不执行任何操作.
     * 此方法执行后，如果对象实现HttpSessionBindingListener，则容器调用HttpSeSessionBindingListener.valueUnbound.
     * 容器然后通知web应用程序中的任何HttpSessionAttributeListener.
     * Removes the object bound with the specified name from
     * this session. If the session does not have an object
     * bound with the specified name, this method does nothing.
     *
     * <p>After this method executes, and if the object
     * implements <code>HttpSessionBindingListener</code>,
     * the container calls
     * <code>HttpSessionBindingListener.valueUnbound</code>. The container
     * then notifies any <code>HttpSessionAttributeListener</code>s in the web
     * application.
     *
     *
     *
     * @param name				the name of the object to
     *						remove from this session
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     */

    public void removeAttribute(String name);





    /**
     * 使用removeAttribute替换此方法.
     * @deprecated 	As of Version 2.2, this method is
     * 			replaced by {@link #removeAttribute}
     *
     * @param name				the name of the object to
     *						remove from this session
     *
     * @exception IllegalStateException	if this method is called on an
     *					invalidated session
     */

    public void removeValue(String name);




    /**
     * 使此会话无效，然后解除绑定到该会话的所有对象.
     * Invalidates this session then unbinds any objects bound
     * to it.
     *
     * @exception IllegalStateException	if this method is called on an
     *					already invalidated session
     *
     */

    public void invalidate();




    /**
     * 如果客户端尚不知道会话或客户端选择不加入会话，则返回true.
     * 例如，如果服务器只使用基于cookie的会话，而客户机禁用了cookies的使用，那么每个请求都会有一个新的会话.
     * 也就是说服务端创建了session，是否已经同步给客户端.
     *
     * Returns <code>true</code> if the client does not yet know about the
     * session or if the client chooses not to join the session.  For
     * example, if the server used only cookie-based sessions, and
     * the client had disabled the use of cookies, then a session would
     * be new on each request.
     *
     * @return 				<code>true</code> if the
     *					server has created a session,
     *					but the client has not yet joined
     *
     * @exception IllegalStateException	if this method is called on an
     *					already invalidated session
     *
     */

    public boolean isNew();



}

